change reader to backend in docs reference to MosaicTiler (#1086)

resolves #1085

Author: hrodmn

docs/src/examples/code/mosaic_from_urls.md

@@ -136,7 +136,7 @@ def DatasetPathParams(url: str = Query(..., description="Dataset URL")) -> List[
     return url.split(",")
 
 
-mosaic = MosaicTiler(reader=MultiFilesBackend, path_dependency=DatasetPathParams)
+mosaic = MosaicTiler(backend=MultiFilesBackend, path_dependency=DatasetPathParams)
 
 ```